Electric power steering apparatus

ABSTRACT

An electric power steering apparatus including a torque sensor to detect a steering torque and a motor control unit to control a motor that applies an assist torque to a steering system of a vehicle, including: a function to switch a control system of the motor between a torque control system to control a motor output torque and a position/speed control system to control a steering angle of a steering in accordance with a predetermined switching trigger. The fade processing time from the torque control system to the position/speed control system and the fade processing time from the position/speed control system to the torque control system are individually set.

TECHNICAL FIELD

The present invention relates to an electric power steering apparatusthat has functions of an automatic steering control (an automaticoperation mode, a parking assist mode, etc.) and a manual steeringcontrol and applies an assist force to a steering system of a vehicle bya motor, especially to an electric power steering apparatus having afunction to switch a control system of the motor between a torquecontrol system to control a motor output torque and a position/speedcontrol system to control a steering angle of a steering.

The present invention further relates to an electric power steeringapparatus where a fade processing time (a gradual changing time) from atorque control (a normal control) to a position/speed control of anautomatic steering and a fade processing time (a gradual changing time)from a position/speed control to a torque control are individually set.

BACKGROUND ART

In an electric power steering apparatus (EPS) which has a motor controlunit and applies a steering assist force to a steering system of avehicle by a rotational force of a motor, the steering assist force isapplied to a steering shaft or a rack shaft by a transmission mechanismsuch as gears and a belt with driving power of the motor via a reducer.Such the conventional electric power steering apparatus performs afeedback control of a motor current in order to precisely generate atorque of steering assist force. The feedback control is to adjust anapplied voltage to the motor such that a difference between a steeringassist command value (current commanded value) and a detected value ofthe motor current becomes small. Adjustment of the applied voltage tothe motor is generally performed by adjusting a duty in a pulse widthmodulation (PWM) control.

Explaining a general configuration (column system) of an electric powersteering apparatus with illustration in FIG. 1, a column shaft (steeringshaft) 2 of a handle (steering wheel) 1 is connected to steered wheels8L and 8R via reduction gears 3, universal joints 4 a and 4 b, a pinionand rack mechanism 5, tie rods 6 a and 6 b, and hub units 7 a and 7 b. Acolumn shaft 2 is provided with a torque sensor 10 that detects asteering torque Ts of the handle 1. A motor 20 that assists the steeringforce of the handle (steering wheel) 1 is connected to the column shaft2 via the reduction gears 3. Electric power is supplied to a controlunit (ECU) 30 for controlling the electric power steering apparatus froma battery 13, and an ignition key signal is inputted into the controlunit 30 through an ignition key 11. The control unit 30 calculates asteering assist command value of an assist (steering assist) commandbased on the steering torque Ts detected by the torque sensor 10 and avehicle speed Vs detected by the vehicle speed sensor 12 and controls acurrent to be supplied to the motor 20 by a voltage control value Vref,which is the steering assist command value after compensation or otherprocessing. Note that a steering angle sensor 14 is not a requirementand may not be disposed. The steering angle may be acquired by arotational sensor connected to the motor 20.

The control unit 30 is connected with a controller area network (CAN) 40that receives various information of the vehicle and the vehicle speedVs can be received from the CAN 40. The control unit 30 may also beconnected with a Non-CAN 41 that receives communication, ananalog/digital signal, radio waves, or others that are different fromthose received by the CAN 40.

In such an electric power steering apparatus, the control unit 30 mainlyincludes a CPU (including an MPU, an MCU, etc.). Functions executed by aprogram inside the CPU are illustrated as an exemplary configuration asillustrated in FIG. 2.

Functions and operations of the control unit 30 are described withreference to FIG. 2. The steering torque Ts from the torque sensor 10and the vehicle speed Vs from the vehicle speed sensor 12 are inputtedinto a current command value calculating section 31. The current commandvalue calculating section 31 calculates a current command value Iref1based on the steering torque Ts and the vehicle speed Vs using an assistmap or the like. The calculated current command value Iref1 is addedwith a compensation signal CM for improving characteristics from acompensating section 34 at an adding section 32A. The current commandvalue Iref2 after addition is limited of the maximum value thereof at acurrent limiting section 33. The current command value Irefm limited ofthe maximum value is inputted into a subtracting section 32B, whereat adetected motor current value Im is subtracted from the current commandvalue Irefm.

The subtraction result I (=Irefm−Im) at the subtracting section 32B isproportional and integral (PI)-controlled at a PI-control section 35.The PI-controlled voltage control value Vref is inputted into aPWM-control section 36, whereat a duty thereof is calculated. The motor20 is PWM-driven by an inverter 37 with a PWM signal calculated theduty. The motor current value Im of the motor 20 is detected by a motorcurrent detection means 38 and is inputted into the subtracting section32B for the feedback.

The compensating section 34 adds a self aligning torque (SAT) 34-3detected or estimated and an inertia compensation value 34-2 at anadding section 34-4. The addition result is further added with aconvergence control value 34-1 at an adding section 34-5. The additionresult is inputted into the adding section 32A as the compensationsignal CM, thereby to improve the characteristics.

In such an electric power steering apparatus, vehicles that have anautomatic steering assist function (an automatic operation, a parkingassist, etc.) and switches between the automatic steering control andthe manual steering control have emerged in recent years. The vehicleshaving the automatic steering assist function perform the automaticsteering control to set a target steering angle based on data from acamera (image), a distance sensor, or other apparatus and to cause anactual steering angle to follow the target steering angle.

In the automatic operation, environment surrounding the vehicle isrecognized based on information from a radar, a camera, an ultrasonicsensor or the like and a steering angle command value that allows forsafely guiding the vehicle is outputted. The electric power steeringapparatus is capable of the automatic operation by performing a positioncontrol of the actual steering angle in such a manner as to follow thesteering angle command value.

In the known electric power steering apparatus having the functions ofthe automatic steering control and the manual steering control in therelated art, for example a back-in parking or a parallel parking isautomatically performed by controlling an actuator (motor) based onrelationship between a pre-stored traveling distance of the vehicle anda turning steering angle.

That is, an automatic steering control apparatus recognizes a parkingspace from a positioning sensor such as an around-view monitor or anultrasonic sensor and outputs a steering angle command value to theEPS-side. The EPS performs a position-control on the actual steeringangle in such a manner as to follow the steering angle command value. Asa result of this, the vehicle is guided into the parking space.

FIG. 3 is a diagram illustrating a control system of an electric powersteering apparatus having the automatic steering control function. Anautomatic steering command unit 50 is inputted with various data from acamera and a positioning sensor (ultrasonic sensor or the like). Asteering angle command value θtc for automatic steering is inputted intoa position/speed control section 51 in an EPS-actuator function via aCAN or the like, and an automatic steering execution command is inputtedinto an automatic steering execution judging section 52 in theEPS-actuator function via the CAN or the like. The steering torque Ts isfurther inputted into the automatic steering execution judging section52. An actual steering angle θr from the EPS-sensor is inputted into theposition/speed control section 51 and a judgment result from theautomatic steering execution judging section 52 is inputted into atorque command value gradual-change switching section 54. Further, thesteering torque Ts from the EPS-sensor is inputted into a torque controlsection 53 in an EPS-power assist function, and a steering assist torquecommand value Tc from the torque control section 53 is inputted into thetorque command value gradual-change switching section 54. Aposition/speed control torque command value Tp from the position/speedcontrol section 51 is also inputted into the torque command valuegradual-change switching section 54. According to the judgment result(ON/OFF of the automatic steering command) from the automatic steeringexecution judging section 52, the steering assist torque command valueTc and the position/speed control torque command value Tp are switchedand output as a motor torque command value, thereby performing thedrive-control of the motor via a current control system.

In this manner, a normal power assist is subjected to a torque controlsystem. Meanwhile, the automatic operation such as the parking assist issubjected to a position/speed control system of the steering angle orother parameters. There are problems such as that the control torquevaries upon the switching between the torque control and theposition/speed control, thereby making the switching over not smooth andthat an unintentional self-steer occurs by a trigger due to variationsin the torque upon the switching over.

To handle such problems, a conventional method to gradually change(gradual-change) the control torque in the torque control and theposition/speed control is used in order to mitigate the torquevariations. For example in Japanese Unexamined Patent Publication No.2004-17881 A (Patent Document 1), when an automatic steering mode isreleased at a time point t0 as illustrated in FIG. 4, “Sθ=OFF” is resetand thereafter an angle control ratio μ is monotonously reduced within apredetermined time ΔT. This allows a command value of a current to beconducted in a motor not to drastically vary even upon switching betweenthe control systems.

THE LIST OF PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Unexamined Patent Publication No.    2004-17881 A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Upon switching between the torque control and the position/speedcontrol, however, the above effect cannot be fully exercised. Thisreason is because there are cases where assisting in an oppositedirection occurs upon switching over to the normal power assist controlsince the position/speed control assists torque in such a manner as tosuppress an external disturbance in a system such as the electric powersteering that allows for inputting the external disturbance from thehandle.

In this manner, the method to gradually change (gradual-change) thecontrol torque in the torque control and the position/speed control isconventionally used in order to mitigate the torque variations. However,this cannot fully exercise its effect in the case of switching betweenthe torque control and the position/speed control.

The present invention has been devised in consideration to the abovecircumstances with an object to provide an electric power steeringapparatus capable of smoothly switching the control systems withoutself-steer by gradually changing a control torque of the torque controland a command value of the position/speed control upon fade processing(gradual-change processing) that switches the control systems.

Means for Solving the Problems

The present invention relates to an electric power steering apparatusincluding a torque sensor to detect a steering torque and a motorcontrol unit to control a motor that applies an assist torque to asteering system of a vehicle, the above-described object of the presentinvention is achieved by that comprising: a function to switch a controlsystem of the motor between a torque control system to control a motoroutput torque and a position/speed control system to control a steeringangle of a steering in accordance with a predetermined switchingtrigger.

Further, the above-described object of the present invention is moreeffectively achieved by that wherein the predetermined switching triggeris ON/OFF of an automatic steering command; or wherein the predeterminedswitching trigger is ON/OFF of a switching command given by an internaljudgment of the steering torque; or wherein, when the automatic steeringcommand is turned ON, a fade processing is started and a post-gradualchange steering-angle command value in a position/speed control isgradually changed from an actual steering angle to a steering anglecommand value, and a level of the assist torque in a torque control isgradually changed from 100% to 0% and then the position/speed controlsystem is operated; or wherein, when the automatic steering command isturned OFF, a fade processing is started and a post-gradual changesteering-angle command value in a position/speed control is graduallychanged from a steering angle command value to an actual steering angle,and a level of the assist torque in a torque control is graduallychanged from 0% to 100% and then the torque control system is operated,or wherein the post-gradual change steering-angle command value in theposition/speed control is gradually changed by an exponential curve, andthe level of the assist torque is gradually changed linearly; or whereina fade characteristic of the fade processing can be freely tuned; orwherein a fade processing time 1 from the torque control system to theposition/speed control system and a fade processing time 2 from theposition/speed control system to the torque control system aredifferent; or wherein the fade processing time 2 is shorter than thefade processing time 1; or wherein the predetermined switching triggeris performed by an automatic steering execution judging section; orwherein the automatic steering execution judging section comprises: acalculating section to calculate an angular speed and an angularacceleration by inputting a steering angle command value; a map judgingsection to judge each of the steering angle command value, the angularspeed and the angular acceleration with a judging map corresponding to avehicle speed; and a diagnosing section to diagnose based on a judgementresult from the map judging section; or further comprising an externaldisturbance observer to compensate inertia and friction of a handle; orwherein the external disturbance observer estimates anexternal-disturbance estimation torque from a difference between anoutput of a steering inverse model of the steering system and an outputof an LPF to limit a band; or wherein values of inertia and friction ofthe steering system are greater than or equal to values of inertia andfriction of the steering inverse model, respectively.

Effects of the Invention

According to the electric power steering apparatus of the presentinvention, a post-gradual change steering angle command value isgradually changed from the actual steering angle to the steering anglecommand value and the actual steering angle is subjected to theposition/speed control in such a manner as to follow the post-gradualchange steering angle command value. This allows the torque commandvalue in the position/speed control to be changed automatically andsmoothly, thereby providing a soft handling feeling to a driver.

Further, even when excessive variations in the steering torque occurupon a fade processing of switching from the automatic steering to thetorque control, the steering torque variations are automaticallycompensated by the position/speed control since the post-gradual changesteering angle command value is gradually changed from the steeringangle command value to the actual steering angle. Therefore, it ispossible to suppress such a failure as to losing the control of thehandle by the driver.

Moreover, as compared to a fade processing time to perform the fadeprocessing from the torque control of the normal steering to theposition control of the automatic steering, a fade processing time fromthe position control to the torque control is shorter. Therefore, thereare advantages that the control is switched over relatively slowly sothat the driver does not feel uncomfortable upon the fade processingfrom the torque control to the position/speed control and that thecontrol can be switched over in a short time and intention of the drivercan be promptly conveyed for avoiding danger upon the fade processingfrom the position/speed control to the torque control.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a configuration diagram illustrating an overview of anelectric power steering apparatus (column system);

FIG. 2 is a block diagram illustrating an exemplary configuration of acontrol system of the electric power steering apparatus;

FIG. 3 is a block diagram illustrating an exemplary configuration of acontrol system of the electric power steering apparatus having a parkingassist mode (automatic steering) function;

FIG. 4 is a characteristic diagram illustrating operation system of aconventional electric power steering apparatus;

FIG. 5 is a configuration diagram illustrating an overview of anelectric power steering apparatus (single pinion system);

FIG. 6 is a configuration diagram illustrating an overview of anelectric power steering apparatus (dual pinion system);

FIG. 7 is a configuration diagram illustrating an overview of anelectric power steering apparatus (dual pinion system (exemplaryvariation));

FIG. 8 is a configuration diagram illustrating an overview of anelectric power steering apparatus (coaxial rack system);

FIG. 9 is a configuration diagram illustrating an overview of anelectric power steering apparatus (rack offset system);

FIG. 10 is a block diagram illustrating an exemplary configuration ofthe present invention;

FIG. 11 is a block diagram illustrating an exemplary configuration of anautomatic steering execution judging section;

FIGS. 12A to 12C are characteristic diagrams illustrating exemplaryjudging maps (steering angle command value, angular speed and angularacceleration);

FIG. 13 is a diagram illustrating relationship between an example ofmounting sensors and an actual steering angle used in the presentinvention;

FIG. 14 is a flowchart illustrating exemplary operations of the presentinvention;

FIG. 15 is a flowchart illustrating a part of exemplary operations ofthe automatic steering judging section;

FIG. 16 is a timing chart illustrating exemplary operations of thepresent invention;

FIGS. 17A and 17B are characteristic diagrams for explaining effects(fade processing) of the present invention;

FIGS. 18A and 18B are characteristic diagrams for explaining effects(fade processing) of the present invention;

FIG. 19 is a timing chart illustrating exemplary operations of anotherembodiment of the present invention;

FIG. 20 is a block diagram illustrating an exemplary configuration of anexternal disturbance observer; and

FIGS. 21A and 21B are characteristic diagrams illustrating exemplaryeffects of providing the external disturbance observer.

MODE FOR CARRYING OUT THE INVENTION

In a conventional torque gradual-change control in the electric powersteering apparatus, there are problems such as that control is notsmoothly switched upon switching between a torque control and aposition/speed control and that unintentional self-steer occurs. In thepresent invention, therefore, a processing that smoothly switches thecontrol without self-steer is implemented by gradually changing acontrol torque of a torque control and a command value of aposition/speed control.

The present invention includes a function to switch control systems of amotor between a torque control system to control a motor output torqueand a position/speed control system to control a steering angle uponsteering in accordance with a predetermined switching trigger (e.g. anautomatic steering command) and implements smooth a fade processingwithout self-steer.

Further, in the present invention as compared to a fade processing time(e.g. 500 to 1000 [ms]) to perform a fade processing from the torquecontrol of the normal steering to the position control of the automaticsteering, a fade processing time (e.g. 20 to 100 [ms]) from the positioncontrol to the torque control is set shorter. Accordingly, the controlis switched over relatively slowly so that a driver does not feeluncomfortable upon the fade processing from the torque control to theposition/speed control and the control can be switched over in a shorttime and intention of the driver can be promptly conveyed for avoidingdanger upon the fade processing from the position/speed control to thetorque control.

Furthermore, the present invention provides an external disturbanceobserver to compensate inertia or friction of the handle and thereforethis allows a driver to easily intervene in the automatic steering bysteering.

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings.

The present invention may be applied to, other than the column systemshown in FIG. 1, a single pinion system a schematic configuration ofwhich is shown in FIG. 5, a dual pinion system an overview of which isshown in FIG. 6, a dual pinion system (exemplary variation) a schematicconfiguration of which is shown in FIG. 7, a rack coaxial system anoverview of which is shown in FIG. 8, and a rack offset system anoverview of which is shown in FIG. 9. The below descriptions will begiven on the column system.

FIG. 10 is a diagram illustrating an exemplary configuration of thepresent invention, and the steering torque Ts is inputted into a torquecontrol section 102 and an automatic steering execution judging section120. A steering assist torque command value Tc from the torque controlsection 102 is inputted into a torque gradual-changing section 103. Asteering angle command value θtc from a CAN or the like is inputted intothe automatic steering execution judging section 120, the steering anglecommand value θt after calculation processing at the automatic steeringexecution judging section 120 is inputted into a steering-angle commandvalue gradual-changing section 100, and a post-gradual changesteering-angle command value θm from the steering-angle command valuegradual-changing section 100 is inputted into a position/speed controlsection 101 together with an actual steering angle θr. A steering-assisttorque command value Tg after the torque gradual-change and aposition/speed control torque command value Tp from the position/speedcontrol section 101 are inputted into an adding section 104 and theaddition result from the adding section 104 is outputted as a motortorque command value. The motor torque command value is inputted into acurrent control system 130, and a motor 131 is driven and controlledthrough the current control system 130.

The automatic steering execution judging section 120 outputs ON/OFF ofthe automatic steering command being a judgment (diagnosis) result. The“ON/OFF” of the automatic steering command is inputted into the torquegradual-changing section 103 and the steering-angle command valuegradual-changing section 100.

The automatic steering execution judging section 120 has a configurationas illustrated in FIG. 11, the steering angle command value θtc isinputted into a calculating section 121, and the calculating section 121calculates an angular speed ωtc and an angular acceleration αtc based onthe steering angle command value θtc. The angular speed ωtc and theangular acceleration αtc are inputted into a map judging section 122 tojudge using a judging map. The map judging section 122 is also inputtedwith the steering angle command value θtc and a vehicle speed Vs. Themap judging section 122 includes a judging map #1 for a steering anglecommand value θtc having a characteristic A1 or B1 as shown in FIG. 12A,a judging map #2 for an angular speed ωtc having a characteristic A2 orB2 as shown in FIG. 12B, and a judging map #3 for an angularacceleration αtc having a characteristic A3 or B3 as shown in FIG. 12C.

The characteristic of the judging map #1 with respect to the steeringangle command value θtc is at a constant value θtc₀ until a vehiclespeed Vs₁ of a low speed and decreases as the characteristic A1 or B1 ina range more than or equal to the vehicle speed Vs₁. The characteristicof the judging map #2 with respect to the angular speed ωtc is at aconstant value ωc₀ until a vehicle speed Vs₂ of a low speed anddecreases as the characteristic A2 or B2 in a range more than or equalto the vehicle speed Vs₂. Further, the characteristic of the judging map#3 with respect to the angular acceleration αtc is at a constant valueαc₀ until a vehicle speed Vs₃ of a low speed and decreases as thecharacteristic A3 or B3 in a range more than or equal to the vehiclespeed Vs₃. Any of the characteristics of the judging maps #1 to #3 canbe tuned, and the characteristic may linearly decrease.

The map judging section 122 judges whether the steering angle commandvalue θtc exceeds the range of characteristic values of the judging map#1, whether the angular speed ωtc exceeds the range of characteristicvalues of the judging map #2, and further whether the angularacceleration αtc exceeds the range of characteristic values of thejudging map #3. A judgment result MD is inputted into a diagnosingsection 123. The diagnosing section 123 outputs “ON/OFF” of theautomatic steering command based on a diagnosis result by time or times(number) and “ON/OFF” of the automatic steering command is also inputtedinto an output section 124. The output section 124 outputs the steeringangle command value θt only when the automatic steering command is “ON”.

Although the steering angle command value θt is inputted into thesteering-angle command value gradual-changing section 100 together withthe actual steering angle θr, the actual steering angle θr is calculatedin the following manner in the present invention.

In a mechanism including a torsion bar 23, for example a sensor asillustrated in FIG. 13 is mounted to a column shaft 2 (2A (input side)and 2B (output side)) and thereby the steering angle is detected. Thatis, the input shaft 2A on a handle 1 side of the column shaft 2 ismounted with a Hall IC sensor 21 as an angle sensor and a 20° rotorsensor 22 for a torque sensor input-side rotor. The Hall IC sensor 21outputs an AS_IS angle θh with 296° period. The 20° rotor sensor 22mounted on the handle 1 side with respective to the torsion bar 23outputs a column input-side angle θs with 20° period and the columninput-side angle θs is inputted into a steering angle calculatingsection 132. The output shaft 2B on the column shaft 2 is mounted with a40° rotor sensor 24 for a torque sensor output-side rotor. The 40° rotorsensor 24 outputs a column output-side angle Go and the columnoutput-side angle Go is inputted into the steering angle calculatingsection 132. The column input-side angle θs and the column output-sideangle signal Go are both calculated into an absolute angle by thesteering angle calculating section 132. The steering angle calculatingsection 132 then outputs a steering angle θr on the column input-sideand a steering angle θr1 on the column output-side of an absolute value.

Although the present invention descriptions are given assuming that thesteering angle θr on the column input-side is the actual steering angle,the steering angle θr1 on the column output-side may be used as theactual steering angle.

Exemplary operations in such a configuration will be described withreference to flowcharts in FIG. 14 and FIG. 15 and a timing chart inFIG. 16.

When the automatic steering command is not “ON” (Step S1), the normalsteering with the assist torque level of 100%, that is, the torquecontrol is performed (Step S15). Then, when the automatic steeringexecution judging section 120 turns “ON” the automatic steering commandat a time point t2 (Step S1), a fade processing of the EPS is startedfrom the time point t2 (Step S2). At this time, “ON/OFF” of theautomatic steering command is outputted from the automatic steeringexecution judging section 120, and the steering angle command valuegradual-changing section 100 gradually changes the post-gradual changesteering-angle command value θm of the position/speed control from theactual steering angle θr to the steering angle command value θt (StepS3). In the torque control of the normal control, the torquegradual-changing section 103 gradually changes the torque level from100% to 0% (Step S4), thereafter the above operations are repeated untilthe fade processing ends (Step S5).

As well, the command value gradual-change of the position/speed controland the level gradual-change of the torque control in a fade section (agradual-change time) may be in any order.

At and after a time point t3 when the fade processing ends, the torquecontrol is switched to the automatic steering (the position/speedcontrol) and then the automatic steering is continued (Step S6).

Thereafter, when the automatic steering command is turned “ON” (a timepoint t4), or when a driver steers the handle during the automaticsteering such that the steering torque Ts exceeds a certain thresholdand the automatic steering command is turned “OFF” (the time point t4),the automatic steering is completed (Step S10) and the fade processingis started (Step S11). Also in this case, “OFF” of the automaticsteering command is outputted from the automatic steering executionjudging section 120. In this way, the steering-angle command valuegradual-changing section 100 gradually changes the post-gradual changesteering-angle command value θm of the position/speed control from thesteering angle command value θt to the actual steering angle θr (StepS12) and the torque gradual-changing section 103 gradually changes thetorque level from 0% to 100% (Step S13). This fade processing iscontinued until a time point t5 (Step S14). At and after the time pointt5 when the fade processing ends, the automatic steering is switched tothe torque control of the normal steering (Step S15).

Note that, a fading characteristic of the steering angle command valuein the position/speed control is represented by an exponential curvewhile the torque gradual-change in the torque control is represented bya linear line in FIG. 16, however, these may be freely tuned accordingto handling feeling. Further, a term between the time point t3 and thetime point t4 in FIG. 16 is an automatic steering section with adeviation “0”.

Exemplary operations of the automatic steering execution judging section120 is as shown in the flowchart of FIG. 15. The calculating section 121in the automatic steering execution judging section 120 is inputted withthe steering angle command value θtc from the CAN or the like (Step S20)and calculates the angular speed ωtc and the angular acceleration αtcbased on the steering angle command value θtc (Step S21). The angularspeed ωtc and the angular acceleration αtc are inputted into the mapjudging section 122, and the vehicle speed Vs is also inputted into themap judging section 122 (Step S22). The map judging section 122 firstjudges whether the steering angle command value θtc corresponding to thevehicle speed Vs is within the range of the characteristic values of thejudging map #1 shown in FIG. 12A, that is, whether the steering anglecommand value θtc is below the characteristic line in FIG. 12A (StepS23). If the steering angle command value θtc is within the range of thecharacteristic values of the judging map #1, next whether the angularspeed ωtc corresponding to the vehicle speed Vs is within the range ofthe characteristic values of the judging map #2 shown in FIG. 12B, thatis, whether the angular speed ωtc is below the characteristic line inFIG. 12B is then judged (Step S24). If the angular speed ωtc is withinthe range of the characteristic values of the judging map #2, whetherthe angular acceleration αtc corresponding to the vehicle speed Vs iswithin the range of the characteristic values of the judging map #3shown in FIG. 12C, that is, whether the angular speed ωtc is below thecharacteristic line in FIG. 12C is then judged (Step S25). If all of thejudging targets are within the range of the respective characteristicvalues, the automatic steering execution judging section 120 turns “ON”the automatic steering command (Step S31) and outputs the steering anglecommand value θtc as the steering angle command value θt for inputtingto the steering-angle command value gradual-changing section 100 (StepS32).

Further, when the steering angle command value θtc corresponding to thevehicle speed Vs is not within the range of the characteristic values ofthe judging map #1 shown in FIG. 12A at the above Step S23, or theangular speed ωtc corresponding to the vehicle speed Vs is not withinthe range of the characteristic values of the judging map #2 shown inFIG. 12B at the above Step S24, or the angular acceleration αtccorresponding to the vehicle speed Vs is not within the range of thecharacteristic values of the judging map #3 shown in FIG. 12C at theabove Step S25, the diagnosing section 123 compares the number of timeswhen the range is exceeded to a predetermined threshold number of timesor compares the length of a time period when the range is exceeded to apredetermined threshold period of time (Step S30). Then, when they donot exceed the thresholds, the operation skips to the above Step S31,where the automatic steering command is turned “ON”. When the number oftimes or the length of the time period exceeds the thresholds, theautomatic steering command is turned “OFF” (Step S33), and the steeringangle command value θt is blocked and is not outputted (Step S34).

As well, the order of the aforementioned Steps S23 to S25 may be changedas appropriate.

When the automatic steering command is turned “ON” as shown in FIGS. 17Aand 17B (a time point t10), the fade processing is started. Thepost-gradual change steering-angle command value θm is gradually changedfrom the actual steering angle θr to the steering angle command valueθt. The actual steering angle θr is position/speed-controlled in such amanner as to follow the post-gradual change steering-angle command valueθm. Consequently, it is possible to automatically and smoothly changethe torque command value of the position/speed control, therebyproviding the soft handling feeling to the driver.

As well, FIG. 17B shows that a deviation in position is represented astorque.

On the other hand, even when the excessive variations in the steeringtorque occur after a time point t21 upon the fade processing of theswitching from the automatic steering to the torque control (a timepoint t20) as shown in FIGS. 18A and 18B, the excessive variations inthe steering torque is automatically compensated by the position/speedcontrol since the post-gradual change steering-angle command value θm isgradually changed from the steering angle command value θt to the actualsteering angle θr. This prevents the driver from losing control of thehandle. That is, as shown in FIG. 18A, in the present invention, sincethe actual steering angle θr is position/speed-controlled in such amanner as to follow the post-gradual change steering-angle command valueθm, an occurrence of a peak is delayed and the position/speed controltorque command value Tp is generated according to a difference betweenthe post-gradual change steering-angle command value θm and the actualsteering angle θr, and then smoothly converges. In the conventionalcontrol, however, since the gradual-change starts from a peak of thetorque as shown in a broken line in FIG. 18A, the convergence is notsmooth. Moreover, a position θr where torque (acceleration) isintegrated twice has a trace as in the broken line shown in FIG. 18A andthe handle thus moves more.

Furthermore, in another embodiment of the present invention, as comparedto the fade processing time (e.g. 1000 [ms]) to perform the fadeprocessing from the torque control of the normal steering to theposition/speed control of the automatic steering, the fade processingtime (e.g. 100 [ms]) from the position/speed control to the torquecontrol is set shorter as shown in FIG. 19. As a result of this, thecontrol is switched over relatively slowly so that the driver does notfeel uncomfortable upon the fade processing from the torque control tothe position/speed control, and the control can be switched over in ashort period of time and intention of the driver can be promptlyconveyed for avoiding danger upon the fade processing from theposition/speed control to the torque control.

In the present invention as further shown in FIG. 20, an externaldisturbance observer 150 to compensate inertia or friction of the handleis provided in the position/speed control section 101 so that a handlemanual-input of the driver is not prevented. Further, the externaldisturbance observer 150 also functions as a torque sensor that detectsthe handle manual-input at a high speed by estimating a torque input bythe driver based on the motor current.

The position/speed control section 101 in FIG. 10 comprises aposition/speed feedback controller 170 and the external disturbanceobserver 150 illustrated in FIG. 20. That is, an input of theposition/speed control section 101 is the post-gradual changesteering-angle command value θm and an output therefrom is theposition/speed control torque command value Tp, and state feedbackvariables are the steering angle θr and the steering angular speed ωr.The position/speed feedback controller 170 comprises a subtractingsection 171 to obtain a steering angle deviation between thepost-gradual change steering-angle command value θm and the steeringangle θr, a position controller 172 to position-control the steeringangle deviation, a subtracting section 173 to obtain a speed deviationbetween the angular speed from the position controller 172 and thesteering angular speed ωr, and a speed controller 174 to speed-controlthe speed deviation. An output from the speed controller 174 isadding-inputted into a subtracting section 154 in the externaldisturbance observer 150. Further, the external disturbance observer 150comprises a steering inverse model 151 of a controlled object that isrepresented by a transfer function “(J₂·s+B₂)/(τ·s+1)”, a low passfilter (LPF) 152 of a transfer function “1/(τ·s+1)” that is inputtedwith the position/speed control torque command value Tp and limits aband thereof, a subtracting section 153 to obtain anexternal-disturbance estimation torque Td*, and a subtracting section154 to output the position/speed control torque command value Tp bysubtraction.

A steering system 160 subjected to the controlled object comprises anadding section 161 to add an unknown external disturbance torque Td tothe position/speed control torque command value Tp, a steering system162 represented by a transfer function “1/(J₁·s+B₁)”, and an integralsection 163 to integrate (1/s) the angular speed ωr from the steeringsystem 162 and to output the steering angle θr. The steering angularspeed ωr is fed back to the position/speed feedback controller 170 andis also inputted into the integral section 163. The steering angle θr isfed back to the position/speed feedback controller 170.

The symbol “J₁” in the transfer function represents the inertia in thesteering system 162, “B₁” represents the friction in the steering system162, “J₂” represents the inertia in the steering inverse model 151, “B₂”represents the friction in the steering inverse model 151, and “τ”represents a predetermined time constant. These have relationshipsrepresented by the following equations 1 and 2.

J ₁ ≧J ₂  (Equation 1)

B ₁ ≧B ₂  (Equation 2)

The external disturbance observer 150 estimates the unknown externaldisturbance torque Td base on a difference between outputs of thesteering inverse model 151 and the LPF 152 and obtains theexternal-disturbance estimation torque Td* as an estimation value. Theexternal-disturbance estimation torque Td* is subtracting-inputted intothe subtracting section 154, and it is possible to realize a robustposition/speed control by subtracting the external-disturbanceestimation torque Td* from an output of the speed controller 174.However, the robust position/speed control results in contradiction thatthe handle cannot be stopped even with intervention by the driver. Inorder to improve this point, the inertia J₂ and the friction B₂ smallerthan or equal to the inertia J₁ and the friction B₁, respectively, whichthe steering system 162 actually has, are inputted as the steeringinverse model 151. As a result of this, the inertia and the friction ofthe handle that the driver feels becomes seemingly smaller. This allowsthe driver to easily intervene in the automatic steering by steering.

Moreover, by monitoring the external-disturbance estimation torque Td*in the external disturbance observer 150, it is possible to detect thesteering torque of the driver instead of the torque sensor. Especially,when the torque sensor uses digital signals, detection of steeringintervention by the driver may be delayed due to influence ofcommunication delay or other reasons. Similarly to the torque sensor,when the external-disturbance estimation torque Td* exceeds a thresholdvalue for a predetermined period of time, the steering intervention maybe determined to be performed and the fade processing may be performed.

FIGS. 21A and 21B are diagrams illustrating characteristics of the angleand the torque, respectively, in the fade processing from theposition/speed control to the torque control when the externaldisturbance observer 150 is provided. The driver turns the handle in anopposite direction to a direction of the steering angle command value θtin the automatic operation and releases the handle when the automaticsteering is turned “OFF” (the fade processing is started). In FIGS. 21Aand 21B, the characteristics of the external disturbance observer 150 ina case where the inertia and the friction satisfy “J₁>J₂” and “B₁>B₂”and a case where “J₁=J₂” and “B₁=B₂” are satisfied are illustrated. FIG.21A is a diagram illustrating exemplary variations in the actualsteering angle θr when the external disturbance observer 150 isprovided. FIG. 21B is a diagram illustrating exemplary variations in thesteering torque Ts and the position/speed control torque command valueTp when the external disturbance observer 150 is provided.

Providing the external disturbance observer 150 allows for providing asmoother operation feeling, thereby enabling switching control at a highspeed. Smaller inertia and friction facilitate the steeringintervention.

EXPLANATION OF REFERENCE NUMERALS

-   1 handle (steering wheel)-   2 column shaft (steering shaft, handle shaft)-   10 torque sensor-   12 vehicle speed sensor-   20, 131 motor-   30 control unit (ECU)-   40 CAN-   41 Non-CAN-   50 automatic steering command unit-   51, 101 position/speed control section-   52, 120 automatic steering execution judging section-   53 torque control section-   54 torque command value gradual-change switching section-   100 steering-angel command value gradual-changing section-   102 torque control section-   103 torque gradual-changing section-   121 calculating section-   122 map judging section-   123 diagnosing section-   130 current control system-   150 external disturbance observer

1-14. (canceled)
 15. An electric power steering apparatus including atorque sensor to detect a steering torque and a motor control unit tocontrol a motor that applies an assist torque to a steering system of avehicle, comprising: a function to switch a control system of said motorbetween a torque control system to control a motor output torque and aposition/speed control system to control a steering angle of a steeringin accordance with a predetermined switching trigger, wherein, when saidautomatic steering command is turned ON, a fade processing is startedand a post-gradual change steering-angle command value in aposition/speed control is gradually changed from an actual steeringangle to a steering angle command value, and wherein, a level of saidassist torque in a torque control is gradually changed from 100% to 0%and then said position/speed control system is operated.
 16. Theelectric power steering apparatus according to claim 15, wherein saidpredetermined switching trigger is ON/OFF of an automatic steeringcommand.
 17. The electric power steering apparatus according to claim15, wherein said predetermined switching trigger is ON/OFF of aswitching command given by an internal judgment of said steering torque.18. The electric power steering apparatus according to claim 15, whereinsaid predetermined switching trigger is performed by an automaticsteering execution judging section.
 19. The electric power steeringapparatus according to claim 15, further comprising an externaldisturbance observer to compensate inertia and friction of a handle. 20.The electric power steering apparatus according to claim 15, whereinsaid post-gradual change steering-angle command value in saidposition/speed control is gradually changed by an exponential curve, andsaid level of the assist torque is gradually changed linearly.
 21. Theelectric power steering apparatus according to claim 15, wherein a fadecharacteristic of said fade processing can be freely tuned.
 22. Theelectric power steering apparatus according to claim 15, wherein a fadeprocessing time 1 from said torque control system to said position/speedcontrol system and a fade processing time 2 from said position/speedcontrol system to said torque control system are different.
 23. Theelectric power steering apparatus according to claim 22, wherein saidfade processing time 2 is shorter than said fade processing time
 1. 24.The electric power steering apparatus according to claim 18, whereinsaid automatic steering execution judging section comprises: acalculating section to calculate an angular speed and an angularacceleration by inputting a steering angle command value; a map judgingsection to judge each of said steering angle command value, said angularspeed and said angular acceleration with a judging map corresponding toa vehicle speed; and a diagnosing section to diagnose based on ajudgement result from said map judging section.
 25. The electric powersteering apparatus according to claim 19, wherein said externaldisturbance observer estimates an external-disturbance estimation torquefrom a difference between an output of a steering inverse model of thesteering system and an output of an LPF to limit a band.
 26. Theelectric power steering apparatus according to claim 25, wherein valuesof inertia and friction of said steering system are greater than orequal to values of inertia and friction of said steering inverse model,respectively.
 27. An electric power steering apparatus including atorque sensor to detect a steering torque and a motor control unit tocontrol a motor that applies an assist torque to a steering system of avehicle, comprising: a function to switch a control system of said motorbetween a torque control system to control a motor output torque and aposition/speed control system to control a steering angle of a steeringin accordance with a predetermined switching trigger, wherein, when saidautomatic steering command is turned OFF, a fade processing is startedand a post-gradual change steering-angle command value in aposition/speed control is gradually changed from a steering anglecommand value to an actual steering angle, and wherein, a level of theassist torque in a torque control is gradually changed from 0% to 100%and then said torque control system is operated.
 28. The electric powersteering apparatus according to claim 27, wherein said predeterminedswitching trigger is ON/OFF of an automatic steering command.
 29. Theelectric power steering apparatus according to claim 27, wherein saidpredetermined switching trigger is ON/OFF of a switching command givenby an internal judgment of said steering torque.
 30. The electric powersteering apparatus according to claim 27, wherein said predeterminedswitching trigger is performed by an automatic steering executionjudging section.
 31. The electric power steering apparatus according toclaim 27, further comprising an external disturbance observer tocompensate inertia and friction of a handle.
 32. The electric powersteering apparatus according to claim 27, wherein said post-gradualchange steering-angle command value in said position/speed control isgradually changed by an exponential curve, and said level of the assisttorque is gradually changed linearly.
 33. The electric power steeringapparatus according to claim 27, wherein a fade characteristic of saidfade processing can be freely tuned.
 34. The electric power steeringapparatus according to claim 27, wherein a fade processing time 1 fromsaid torque control system to said position/speed control system and afade processing time 2 from said position/speed control system to saidtorque control system are different.
 35. The electric power steeringapparatus according to claim 34, wherein said fade processing time 2 isshorter than said fade processing time
 1. 36. The electric powersteering apparatus according to claim 30, wherein said automaticsteering execution judging section comprises: a calculating section tocalculate an angular speed and an angular acceleration by inputting asteering angle command value; a map judging section to judge each ofsaid steering angle command value, said angular speed and said angularacceleration with a judging map corresponding to a vehicle speed; and adiagnosing section to diagnose based on a judgement result from said mapjudging section.
 37. The electric power steering apparatus according toclaim 31, wherein said external disturbance observer estimates anexternal-disturbance estimation torque from a difference between anoutput of a steering inverse model of the steering system and an outputof an LPF to limit a band.
 38. The electric power steering apparatusaccording to claim 37, wherein values of inertia and friction of saidsteering system are greater than or equal to values of inertia andfriction of said steering inverse model, respectively.